Optical displays using liquid crystal technology have been widely used in products ranging from small devices such as clocks, watches, calculators, portable phones (all of which are normally battery powered) to larger items such as computers and television receivers. Enhancing display contrasts and providing vivid colors and brightness has been an objective of designers. Where power is obtained from a non-battery source (and is therefore unlimited) continuous backlighting is used; in battery powered devices such an approach would quickly drain all battery power.
Several approaches have been used to introduce colors to a display that works under ambient light (back lighting is used only intermittently) and where a battery is used. One is to add a color filter to a front polarizer which allows the unwanted other color components to be absorbed in addition to the light of unwanted polarization. Thus, in a so called positive mode of operation the display pixels show the color of the filter but the background will carry and undesirable tinge of the color on a silvery background which results in poorer contrast.
Another technique is placing a color filter between the rear polarizer and reflector. This allows more incident light to go through the liquid crystal assembly but only a fraction of this light is reflected to the observer on the return trip again resulting in poor contrast.
Lastly, the display contrast may be worsened due to the typical time division multiplexing driving of the pixels. This is normally used to obtain high pixel counts for a fixed number of drivers. As a result, the light available to the liquid crystal display will be limited to a fraction depending on the duty cycle used (usually a small fraction) While not a problem for non-color displays, it is for color.